The Office for Nuclear Regulation (ONR) holds the report.
The information in this document is being withheld as it falls under the exception in Regulation 12(5)(a) of the Regs, because it relates to national security matters.
This is a qualified exception that is subject to the public interest test. This means ONR has to balance the public interest factors favouring disclosure against those favouring non-disclosure.
Openness, transparency and availability of information.
The release could aid the commission of a malicious act and the public would not expect ONR to release information that would aid such an act.
The balance is in favour of limiting the potential for a malicious act, so the report should be withheld.